What is the thickness of the spacer?

[jazzchan]

Dear all,

the question is ...
Two flat glass plates touch at one edge and are separated at the other edge by a spacer. Using vertical viewing and light is 589 nm(wavelength), five dark fringes(D) are obtained from edge to edge. What is the thickness of the spacer ??

i know that the thinkness of space(T) = m(1/2wavelength), but
would u mind to give me some idea for what is the m ?? is 4 or 5 ??

I think it is 5...but i checked the correct answer is 4 ?? why ?? because of 'from edge to edge' ??

best regards and thanks

JazzChan

 

[member 553083]

The answer is 4 because the fringes you are seeing are interference fringes. This means that light from one edge is interfering with light from the other edge, producing constructive and destructive interference. This only happens when the path difference between the two light sources is an integral number of wavelengths. So, the path difference between the two edges must be 4x589nm in order for the fringes to line up. Therefore, the thickness of the spacer must be 4x589nm/2 = 1178nm.

 

[M98Ranger]

Hello JazzChan,

Thank you for your question regarding the interference of light. The value of "m" in the equation T = m(1/2wavelength) represents the number of dark fringes observed between the two plates. In this case, since five dark fringes are observed from edge to edge, the value of m would be 5. However, it is important to note that the equation T = m(1/2wavelength) is only applicable when the light is incident perpendicular to the plates. If the light is at an angle, the value of m may differ.

As for the discrepancy in the answer, it could be due to the wording in the question. Since the question states "from edge to edge," it is possible that the answer is asking for the number of dark fringes observed between the two plates, rather than the total number of dark fringes. In this case, the correct answer would be 4.

I hope this explanation helps clarify any confusion. Best of luck with your studies!